Antenna system and radar system incorporating the same

ABSTRACT

An antenna system comprising an array of antenna elements, the array comprising a plurality of groups of antenna elements wherein each group comprises one or more antenna elements arranged in series, and wherein the system further comprises first phase-control means for performing the function of introducing respective phase-shifts to transmitted or received signals passed to or received from each of said groups to provide beamforming and second phase-control means for performing said function with respect to a sub-set of said groups. An antenna system of the invention allows two radar beam patterns having different spatial characteristics to be generated using a single antenna system. The invention also provides a radar system incorporating an antenna system of the invention.

The invention relates to antenna systems and to radar systems.

Radar systems for certain applications are required to have thecapability to produce more than one radar beam pattern in order toperform more than one type of detection and ranging. For example, anautomotive radar system may be required to produce a long-range beampattern having a relatively narrow angular extent in azimuth for use incruise control on motorways, and also a short-range beam pattern havinga wider angular azimuthal extent for use in a parking sensor system andto monitor nearby vehicles for the purpose of collision avoidance.Existing automotive radar systems of this type use two separate radarsub-systems to provide such beam patterns; see for example “A WidebandMillimetre-Wave Front-End for Automotive Radar” by J. C. E. Mayock etal, 1999 IEEE MTT-S Digest. This approach requires four antennasub-systems if the two radar sub-systems are bistatic. Accommodation offour antenna sub-systems within a vehicle body places significantconstraints on vehicle body shape and/or size. Also, each sub-systemrequires dedicated operating electronics because the transmittedfrequencies of the short- and long-range beam patterns are different.(Typically 24 GHz is used for the short-range beam and 77 GHz for thelong-range beam). The complexity and size of these existing automotiveradar systems results in substantial cost both to manufacturers andconsumers, impeding the take-up of automotive radar technology. Similarproblems arise in other applications where antenna and radar systemshaving more than one beam pattern are required.

A first aspect of the invention provides an antenna system comprising anarray of antenna elements, the array comprising a plurality of groups ofantenna elements wherein each group comprises one or more antennaelements arranged in series, and wherein the system further comprisesfirst phase-control means for performing the function of applyingrespective phase-shifts to transmitted or received signals passed to orreceived from each of said groups to provide beamforming and secondphase-control means for performing said function with respect to asub-set of said groups.

A single antenna system of the invention allows two radar beam patternshaving different spatial characteristics to be produced, thus reducingthe number of antenna systems needed to provide such functionalitycompared to prior art systems. An antenna system of the invention may beused for transmission, reception or both transmission and reception. Byoperating the array, and the sub-set of groups, at substantially thesame frequency the invention allows a reduction in the cost andcomplexity of operating electronics used with the antenna system.

One or more additional sub-sets of groups of antenna elements may bedefined using further respective phase-shifting means, providing for oneor more additional beam patterns to be generated.

Although arrangements of switches and conventional phase-shifters may beused to carry out phase-control, preferably either the first or thesecond phase-control means is a Rotman lens. More preferably, both thefirst and second phase-control means are Rotman lenses. Rotman lensesare described in, inter alia, the proceedings of the 22^(nd)International Communications Systems Satellite Conference & Exhibition2004 (American Institute of Aeronautics & Astronautics), paper AIAA2004-3196 by P. S. Simon, and are advantageous because they can berealised in planar, cheap and reliable form and are less affected byaberration errors than other phase-control devices.

Conveniently, the array of antenna elements is substantiallyrectangular, and each of the groups of antenna elements is a row orcolumn of the array. This provides for simpler manufacture.

In one embodiment, the sub-set of groups of antenna elements is made upof contiguous rows or columns of the array.

Preferably the array of antenna elements is substantially planar,allowing the array to be placed behind a registration plate of avehicle, for example.

For ease of manufacture, preferably the array of antenna elements andthe Rotman lenses are mounted on a common former. For example, theformer may be substantially cuboid in shape, with the first and secondRotman lenses mounted on opposite faces of the cuboid former and thearray of antenna elements mounted on a face of the cuboid formeradjacent of the faces mounting the Rotman lenses. Alternatively theformer may be laminar with the array of antenna elements, and the Rotmanlenses, mounted on the same side, or opposite sides, thereof.Manufacture of an antenna system of the invention is further simplifiedif each antenna element is a patch antenna element.

A second aspect of the invention provides a radar system comprising anantenna system of the invention. For reasons mentioned above, preferablyone of the phase-control means is a Rotman lens, or, more preferably,both of the phase-control means are Rotman lenses. In order to allowrapid directional scanning of the radar pattern of the whole array orthe sub-set of groups (in either transmission or reception), inputs tothe Rotman lenses may be coupled to respective RF switches. A RF signalfor transmission is preferably provided by a monolithic microwaveintegrated circuit (MMIC) operable to provide the signal to either ofthe RF switches, as a MMIC may be integrated with the RF switches.

A radar system of the invention may be a frequency-modulated,continuous-wave (FMCW) radar system. In order to provide low-phase noiseon transmit and highly coherent operation (and hence high rangeaccuracy) at low cost, preferably the radar system comprises an RFoscillator, a direct digital synthesiser (DDS) arranged to provide afrequency-modulated signal and a mixer arranged to mix respectiveoutputs of the RF oscillator and the DDS to provide a FMCW signal fortransmission, wherein the clock signal of the DDS is derived from the RFoscillator. By locking the DDS to the RF oscillator, a free-runningsource (e.g. a free-running dielectric resonator oscillator (DRO)) maybe used because any jitter in the output of the RF oscillator thencorresponds to the same jitter in the DDS clock signal. Additionally, inembodiments of the invention in which one or more analogue-to digitalconverters (ADCs) are provided to digitise received signals, the clocksignal for each ADC is preferably derived from the RF oscillator. Thevarious clock signals may be obtained by frequency-dividing the outputof the RF oscillator. Use of a free-running source as the RF oscillator,rather than a phase-locked source, provides optimum phase noise atoffset frequencies that cause most effect in FMCW radars (typically 100kHz and 1 MHz from the carrier frequency). The use of a DDS as describedabove provides very high frequency-sweep linearity and hence very highrange resolution.

In automotive radar applications it is important to avoid the problem ofinterference between radars systems of vehicles which are adjacent ornearly adjacent and travelling in the same direction, as occurs forexample on motorways when vehicles are moving in adjacent lanes. Ifautomotive radar systems fitted to adjacent vehicles use the samefrequency then each system will return spurious results. To avoid thisproblem, the frequency of radiation transmitted from a radar system ofthe invention is preferably a function of transmission direction.

A radar system of the invention may be monostatic, in which case asingle antenna system of the invention is needed for both transmissionand reception, or bistatic, in which case two antenna systems of theinvention are required. The same operating electronics can be used tooperate the whole array of antenna elements and to operate the sub-setof groups of antenna elements.

Embodiments of the invention are described below by way of example onlyand with reference to the accompanying drawings in which:

FIGS. 1 & 2 show perspective views of an antenna system of theinvention;

FIG. 3 shows an array of patch antenna elements comprised in the antennasystem of FIGS. 1 and 2;

FIG. 4 shows drive electronics for use with the antenna system of FIGS.1 & 2 to provide azimuthal scanning in transmission or reception; and

FIGS. 5 & 6 show advantageous arrangements for generating a transmittedFMCW signal and for digitising radar returns within a radar system ofthe invention.

Referring to FIGS. 1, 2 and 3, an antenna system of the invention isindicated generally by 100. The antenna system 100 comprises asubstantially cuboid former 114 mounting first 108 and second 116 Rotmanlenses on opposite faces thereof and a rectangular array 102 of antennaelements on a face of the former 114 adjacent to the faces mounting theRotman lenses 108, 116. The array 102 comprises 69 groups of patchantenna elements, each group comprising a linear array of 11 elementsconnected in series forming a column of the array 102. Rotman lens 108has 69 outputs each of which is coupled to a respective group of patchantenna elements via a connecting line. The 69 connecting lines areindicated collectively by 106. Rotman lens 116 has 19 outputs, each ofwhich is coupled by a connecting line to a respective column of antennaelements in a sub-set 104 of columnar groups of the array 102, thesub-set being the central 19 columns (shown shaded in FIG. 3) of thearray 102. The 19 connecting lines coupling outputs of the lens 116 torespective columns in the sub-set 104 of groups of antenna elements ofthe array 102 are indicated collectively by 112. Connecting lines to thevarious inputs of the Rotman lenses 108, 116 are indicated by 110, 118respectively.

The antenna system 100 may operate either as a transmitter or areceiver, or both when used in a monostatic radar system. A long-rangebeam pattern with a narrow angular extent in azimuth may be produced byoperating the system 100 so that only the sub-set 104 of groups ofantenna elements are activated. The transmit/receive direction of thesystem in this mode of operation may be varied in azimuth by applying atransmission signal to an appropriate input of lens 116 in the case oftransmission, or by processing a signal from an appropriate input of thelens 116 in the case of reception. The azimuthal direction of ashort-range beam pattern may be varied similarly, i.e. by applying atransmission signal to an appropriate input of lens 108 in the case oftransmission, or by processing signals from an appropriate input of thelens 108 in the case of reception.

FIG. 4 schematically illustrates an arrangement for producing ascanning, transmitted, long-range radar beam pattern using the antennasystem 100. Input lines 118 to the Rotman lens 116 (the outputs 112 ofwhich are connected to the sub-set 104 of groups of the array 102) areconnected via an alumina drop-in unit 120 to an RF switch 122. Amonolithic microwave integrated circuit (MMIC) 124 provides an RFtransmission signal and a direction-scanning signal to the RF switch122. At a particular instant, the RF transmission signal appears on aparticular output of the RF switch 122 according to the status of thedirection-scanning signal. The RF transmission signal is applied to theRotman lens 116 via a particular input 118. Drive signals appear at eachof the outputs 112 of the Rotman lens 116 with appropriate phasing togenerate a plane wave having an azimuthal transmission directioncorresponding to that particular input of the Rotman lens 116. A secondRF switch and drop-in unit (not shown) are provided for the Rotman lens108 to allow transmission of a short-range beam pattern from the wholeof the array 102 of antenna elements. The MMIC 124 is arranged forswitching between the RF switches so that a long- or short-range beammay be transmitted as required.

The arrangement shown in FIG. 4 may also be used for reception. Forexample, a direction-scanning signal applied to RF switch 122 providesfor each of the lines 118 to be coupled through the RF switch in turn,corresponding to scanning of the receive direction in azimuth. Switchingmeans may be provided to allow switching between lenses 108, 116 so thatreception may be carried out using either the whole array 102 or usingthe sub-set 104 of antenna elements, as desired.

Two separate arrangements of the type shown in FIG. 4 may be used (onefor transmission and one for reception) in a bistatic radar system ofthe invention.

FIG. 5 shows an arrangement for providing high range range-accuracy in aFMCW radar incorporating an antenna system of the invention. Afree-running DRO 132 operating at 9200 MHz is coupled to a mixer 134.Output from the DRO 132 is also down-converted by a firstfrequency-divider 136 to provide a clock signal to a DDS 138 which isarranged to output a signal having a frequency which periodicallyincreases from 200 MHz to 250 MHz in a sawtooth form, as shown in theFigure. Mixing of the DDS and DRO outputs at the mixer 134 provides anFM RF transmission signal which also has a sawtooth frequency as afunction of time, sweeping from 9400 MHz to 9450 MHz. Output from thefirst frequency-divider 136 is passed to a second frequency-divider 140which provides a further down-converted signal to a complex programmablelogic device (CPLD) 142 which in turn provides a clock signal to an ADC144 which digitises signals received from the antenna system of theradar system. By locking the various clock signals to a single reference(DRO 132) the reference can be free-running because any jitter in thereference corresponds to the same jitter on the clock signals, thusproviding highly coherent operation.

FIG. 6 show an alternative to the FIG. 5 arrangement, in which theoutput of a mixer 154 is up converted to provide an RF transmissionsignal having a frequency which is an order of magnitude greater thanthat of the arrangement shown in FIG. 5.

1. An antenna system comprising an array of antenna elements, the arraycomprising a plurality of groups of antenna elements wherein each groupcomprises one or more antenna elements arranged in series, and whereinthe system further comprises a first phase-controller arranged toperform the function of applying respective phase-shifts to transmittedor received signals passed to or received from each of said groups toprovide beamforming and a second phase-controller for performing saidfunction with respect to a sub-set of said groups.
 2. An antenna systemaccording to claim 1 wherein at least one of the first and secondphase-controllers comprises a Rotman lens.
 3. An antenna systemaccording to claim 2 wherein both the first and second phase-controllerseach comprise a Rotman lens.
 4. An antenna system according to claim 1wherein the array of antenna elements is substantially rectangular andeach of said groups of antenna elements is a row or column of the array.5. An antenna system according to claim 4 wherein said sub-set of groupsis made up of contiguous rows or columns of the array.
 6. An antennasystem according to claim 5 wherein the array of antenna elements issubstantially planar.
 7. An antenna system according to claim 4 whereinarray of antenna elements and the Rotman lenses are mounted on a commonformer.
 8. An antenna system according to claim 7 wherein the former issubstantially cuboid and the first and second Rotman lenses are mountedon respective opposite faces of the cuboid former and the array ofantenna elements is mounted on a face of the cuboid former adjacent tothe faces mounting the Rotman lenses.
 9. An antenna system according toclaim 7 wherein the former is laminar.
 10. An antenna system accordingto claim 1 wherein each antenna element is a patch antenna element. 11.A radar system comprising an antenna system according to claim
 1. 12. Aradar system according to claim 11 wherein at least one of the first andsecond phase-controllers comprises a Rotman lens.
 13. A radar systemaccording to claim 12 wherein both the first and secondphase-controllers each comprise a Rotman lens.
 14. A radar systemaccording to claim 13 further comprising first and second RF switchescoupled to the first and second Rotman lenses respectively.
 15. A radarsystem according to claim 14 further comprising a monolithic microwaveintegrated circuit (MMIC) operable to provide a transmission signal toeach RF switch.
 16. A radar system according to claim 11 wherein theradar system is a FMCW radar system.
 17. A radar system according claim16 further comprising a RF oscillator, a DDS arranged to provide afrequency-modulated signal and a mixer arranged to mix respectiveoutputs of the RF oscillator and the DDS to provide a FMCW signal fortransmission, and wherein the clock signal of the DDS is derived fromthe Rb oscillator output.
 18. A radar system according to claim 17comprising one or more ADCs arranged to digitise received signals andwherein the clock signal of each ADC is derived from the RF oscillatoroutput.
 19. A radar system according to claim 17 and further comprisinga frequency-divider arranged to frequency-divide the output of the RFoscillator to generate the, or each, clock signal.
 20. A radar systemaccording to claim 17 wherein the RF oscillator is a free-running DRO.21. A radar system according to claim 11 and arranged to produce fromthe antenna array a transmitted signal having a frequency which is afunction of transmission direction.
 22. A radar system according toclaim 11 wherein the radar system is monostatic.
 23. A radar systemaccording to claim 11 wherein the radar system is bistatic. 24-26.(canceled)